The present invention relates to a polygonal mirror used to deflect an incident light ray; the deflected ray is employed to perform a scanning function in a printing device, such as a laser beam printer.
Printing devices such as laser beam printers, photo-plotters etc. have become essential tools for the printing of documents and photographs. These laser beam print devices utilize an electrophotographic system wherein a laser beam is used to scan the photosensitive layer on the surface of a photosensitive drum along the axis thereof (main scanning), and at the same time the photosensitive drum is rotated (auxiliary scanning) to thereby form a latent image corresponding to an image (generated by a computer or optical scanner), onto the photosensitive layer on the surface of the photosensitive drum.
Toner is first deposited to the latent image by a development unit to form a toner image (development), and the toner image is transferred onto a recording paper by a transfer unit. Then, the toner image is fixed onto the recording paper by a fixing unit. The laser photo-plotter scans the surface of a photosensitive material by a laser beam to create an original for a print board and the like. In the laser beam printer, laser photo-plotter etc., a rotary multi-faced mirror or a so-called polygonal mirror is employed such that the laser beam, from a fixedly mounted laser diode, scans linearly. The polygonal mirror uses a plurality of the side surfaces of a regular prism with a regular polygon in a plane figure as deflection mirrors.
Conventionally, the polygonal mirror is formed from a raw material such as optical glass or aluminum alloy. The raw material is processed and formed to a predetermined polygonal shape and the side surfaces (surfaces to be subjected to a mirror finish) thereof are polished to a predetermined accuracy (surface roughness/degree of flatness) and then the sides are coated with silver or aluminum to form mirror surfaces.
In recent years, aluminum alloy, that is easy to process, has been increasingly employed as the raw material for the polygonal mirror. However, although the aluminum alloy is easy to process, it is extremely difficult to process the individual surfaces of the polygonal prism to bring the accuracy to a requisite level. Thus, a long time is required for processing, thereby increasing the manufacturing cost. To cope with this problem, plastic material has been considered as a new raw material for the polygonal mirror. A metal mold could then be used to integrally form the polygonal mirror by means of injection molding.
When the polygonal mirror is molded by using the plastic material as described above, however, the reflection faces thereof must be finished to glossy surfaces with a surface roughness of 0.1 .mu.m or less in order to effectively reflect the light ray. Further, the reflection faces must have a plane surface accuracy of at least one half the wavelength of the light ray so that they can accurately reflect the light ray.
Although various conventionally molding techniques have been proposed to satisfy these conditions, they cannot greatly change the physical property of the plastic material itself. In particular, the molding shrinkage percentage is difficult to change. Thus it is difficult to maintain a surface deformation caused by molding shrinkage within a required accuracy. For example, there has been conventionally employed a method of mixing the plastic material with other material such as carbon fiber, glass fiber etc. to reinforce the plastic material and stabilize its molding dimension. With the conventional method of mixing the fiber, however, a surface gloss and flatness, necessary to utilize the surfaces of the molded plastic material as reflection mirrors, cannot be presently obtained.
Further, when the polygonal mirror is integrally molded by injection molding using the plastic material as described above, the plastic material, being melted, is supplied into the cavity of a metal mold through a runner gate at a predetermined pressure. As a general method, the runner gate is located at a mounting hole portion defined through the central portion of the polygonal mirror in the thickness direction thereof. In this case, since the plastic material, that has cooled and solidified in the runner gate, is not required, it must be cut off in a subsequent process. If even a slight amount of the unrequired part remains after the cutting-off process, a problem arises in that the inside diameter of the mounting hole and the distance from the center of rotation to the respective reflection faces, cannot be kept to predetermined values.
To cope with this problem, a method of cutting off the runner gate in the metal mold, a method of constituting the gate from a pin gate, and the like have been considered, but these methods make the arrangement of the metal mold complex and the molding process difficult, and thus an improved method is desired.
A further problem arises however when the polygonal mirror is integrally formed by means of injection molding by using plastic material, as described above. Because of the polygonal shape, sink marks develop in the portion where the mirror reflection surfaces are formed due to the non-uniformity of the wall thickness and also because the molding shrinkage percentage varies depending upon differences in molding conditions (pressure, temperature, time and operating speed), so that the molding shrinkage percentage sometimes exceeds an allowable accuracy range. Therefore, this makes the plastic material difficult to use and, if employed, will produce a low yield.
Further, when the polygonal mirror is rotated at high speed, there is a possibility that the mirror reflection faces are deformed by the centrifugal force produced by the rotation. Thus, the reflected laser beam may be scanned in an erroneous direction. Furthermore, since all the side surfaces of the polygonal mirror are composed of reflection faces, it is difficult to handle the mirror during the manufacturing process.